In constructing a space truss structure by joining structural members, like steel pipes, to connector nodes, the combination of grids basically having a triangle formed by structural members generally makes a truss structure. As shown in FIG. 17, which indicates a partial plan view of the framework of a dome, some grids 12 in a single layer space frame structure assembled by using structural members 11, 11 are often rectangular or pentagonoid.
The grid 12 having a figure except a triangle must be reinforced by disposing brace members on the diagonal line thereof. The dimensions and the material of a non-pretensioned brace member are chosen appropriately to not only axial compressive force but tensile force acting thereon, similarly to the choice of those of structural pipes.
On the other hand, a small-diameter pretensioned steel rod is available to a brace member in consideration of axial tensile force only acted thereon since the pretension introduced into a brace member contributes to establish a stable grid. It is hereat natural to determine the value of pretension so as not to make the grid deform against the designed load. The pretension of brace member 13 is usually provided by a turnbuckle 14 roughly drawn by a broken line at an intermediate portion of a steel rod.
The brace member 13 is easily disposed between two nodes 2, 2, which are movable and/or unmovable in the truss space, by adjusting the length thereof by means of a turnbuckle.
The turnbuckles installed on the brace members often spoil the appearance of a truss structure, especially, in the case applying them to a framework supporting glazed roof.
It is remarkably difficult to introduce designed pretension into a brace member by a turnbuckle, because an operator can hardly grap exact elongations of one half and another half of the brace member, which are separated by a turnbuckle, during construction work.
U.S. Pat. No. 4,872,779 discloses a joint device for joining a structural member to a connector node. The disclosure teaches to engage a fastening bolt, which is mounted on the edge cover closing the end opening of a steel pipe, to a connecting screw hole in the connector node by rotation of a sleeve covering the fastening bolt.
The joint device described above is not available to the connection of a small-diameter steel rod as a brace member with a connector node since a fastening bolt must be previously inserted in the edge cover of a steel pipe. Furthermore, it is impossible to introduce pretension into a steel pipe by means of such a kind of joint device.